When human erytrocytes (RBC) in vitro interact with bilirubin (B), they become sensitized to hemolysis in visble light. Although this photoreaction occurs in the plasma membrane, little is known on a molecular basis about the modifications in membrane components. During our studies on the possible photodynamic action of B on serum albumin, we began to investigate B-sensitized photodamage to the isolated RBC membrane (ghost), with special interest in the polypeptide constituents. When B-charged ghosts under 02 were irradiated with blue light, large scale polymerization of most of the major proteins occured, as evidenced by SDS-polyacrylamide electrophoresis (Girotti, A.W., manuscript in preparation). A negligible effect was seen in the dark, or in the absence of B or O2. This observation stimulated the following proposed work: 1) Higher resolution analysis (e.g. SDS electrophoresis in agarose-polyacrylamide) of the high molecular weight photoproduct during various stages of the reaction. This may give clues to the proximity not only of certain peptides to one another in the membrane, but also to B. 2) Since lipid peroxidation occurs during the photoreaction, the importance of peroxidation in protein polymerization will be assessed by use of free-radical inhibitors such as alpha-tocopherol. 3) The observed photoinactivation of integral ghost enzymes (e.g. Na ion-ATPase and cholinesterase) will be examined in greater detail and reasons for differing photosensitivity sought. 4) Similar studies with RBC will be started to determine whether the observations with ghosts also apply to the intact cell. Results from this work may have important implications in our understanding of how photosensitizers in general can act on tissue membranes.